Stress-corrosion fatigue cracking of cold expanded components

Fasteners like rivets are widely used for assembling parts in the aircraft industry. The fasteners not only help in easy assembly and dismantling of the parts, but also help in proper distribution of the load applied on the body. However, in order to fasten two parts together, a number of holes have to be drilled on both parts and these holes tend to raise the stress in the region surrounding them. This stress concentration effect of the holes reduces the fatigue life of the part when cyclic load is applied to the part. To offset the stress concentration effect of the holes, a technique called Split Sleeve Cold Expansion has been widely used in the aircraft industry. It induces a compressive residual stress in the fastener hole and the region around it, which retards the growth of fatigue crack and thereby improves the fatigue life of the component.

The objective of this thesis work was to analyze the effect of Split Sleeve Cold Expansion on the fatigue life of AI-7075 T6 specimen subjected to corrosion. In order to fully understand the interaction between cold expansion, corrosion and fatigue life, two possible cases were considered. In the first case the specimens were cold expanded first and then corroded to required mass loss level. In the second case the specimen were corroded prior to cold expansion. The specimens were corroded using a galvanic corrosion cell. The specimens were then tested for fatigue strength.

The results showed that cold expansion improved the fatigue life of un-corroded and mildly corroded specimen by a factor of 20. However, fatigue life of cold expanded specimens dropped exponentially with corrosion. For severely corroded specimens there was no significant improvement in fatigue life due to cold expansion. The results also showed that for specimens subjected to the same degrees of cold expansion and corrosion, the fatigue life of specimens cold expanded after corrosion was lower than the fatigue life of specimens cold expanded prior to corrosion.